LCDs are widely used in various modern information products, such as notebooks, personal digital assistants (PDAs), video cameras and the like. The wide usage of the LCD is due to its advantages such as portability, low power consumption, and low radiation. Because liquid crystal in an LCD does not emit any light itself, a backlight system is usually needed to enable the LCD to display images.
A typical backlight system includes a plurality of backlight lamps, an inverter circuit for driving the backlight lamps, and a backlight control circuit. The backlight control circuit is for feeding back currents of the backlight lamps and protecting the backlight system when an open circuit occurs in any of the backlight lamps.
FIG. 3 is an abbreviated diagram of a conventional backlight control circuit 10. The backlight control circuit 10 is typically installed in a backlight system (not shown). The backlight system is used together with an LCD, both being installed in a product such as a notebook, a PDA, a video camera, etc. The backlight control circuit 10 includes four sampling circuits 11, a pulse width modulation integrated circuit (PWM IC) 13, a feedback circuit 12, and a protecting circuit 14.
Each sampling circuit 11 includes a sampling output 110, a backlight lamp 111, a first diode 113, a second diode 114, and a sampling resistor 115. One terminal of the backlight lamp 111 is electrically coupled to a positive terminal of the first diode 113, and electrically coupled to a negative terminal of the second diode 114. A negative terminal of the first diode 113 is grounded via the sampling resistor 115. A positive terminal of the second diode 114 is grounded directly. The other terminal of the backlight lamp 111 is electrically coupled to a backlight lamp 111 of an adjacent sampling circuit 11 via a respective coupling capacitor 116. Thus a first pair of the sampling circuits 11 are electrically coupled with each other via a first coupling capacitor 116, and a second pair of the sampling circuits 11 are electrically coupled with each other via a second coupling capacitor 116.
The PWM IC 13 includes a current sense pin 130 and a voltage output pin 131. The current sense pin 130 is grounded via an integrating capacitor 15. The voltage output pin 131 outputs a direct current (DC) voltage with a value of 5V. The PWM IC 13 controls the backlight system in which the backlight control circuit 10 is installed. In particular, the PWM IC 13 controls the backlight system to switch to a protecting state, if the current sense pin 130 has a low voltage.
The feedback circuit 12 includes a first feedback resistor 122 and a second feedback resistor 124. The first feedback resistor 122 is electrically coupled between the current sense pin 130 of the PWM IC 13 and one of the sampling outputs 110 of the first pair of sampling circuits 11. The second feedback resistor 124 is electrically coupled between the current sense pin 130 of the PWM IC 13 and one of the sampling outputs 110 of the second pair of sampling circuits 11.
The protecting circuit 14 includes an electronic switch 140, a first transistor 141, a second transistor 142, a third transistor 143, and a fourth transistor 144. Each one of the transistors 141, 142, 143, and 144 corresponds to a respective sampling circuit 11. The electronic switch 140 is an NMOS (negative-channel metal-oxide semiconductor) type transistor. A gate electrode of the electronic switch 140 is electrically coupled to the voltage output pin 131 via a bias resistor 146. A drain electrode of the electronic switch 140 is electrically coupled to the current sense pin 130. A source electrode of the electronic switch 140 is grounded. Gate electrodes of the four transistors 141, 142, 143, and 144 are respectively electrically coupled to the sampling outputs 110 of the sampling circuits 11 via respective third diodes 145. Each of these gate electrodes is also grounded via a respective resistor (not labeled) and a respective capacitor (not labeled) electrically coupled in parallel. A drain electrode of the first transistor 141 is electrically coupled to the electronic switch 140. A source electrode of the first transistor 141 is electrically coupled to a drain electrode of the second transistor 142. A source electrode of the second transistor 142 is electrically coupled to a drain electrode of the third transistor 143. A source electrode of the third transistor 143 is electrically coupled to a drain electrode of the fourth transistor 144. A source electrode of the fourth transistor 144 is grounded.
Operation of the backlight control circuit 10 is as follows. When the backlight lamps 111 of the sampling circuits 11 are in normal working states, the sampling outputs 110 provide high voltages to the gate electrodes of the corresponding transistors 141, 142, 143, and 144 via the third diodes 145. The high voltages cause the four transistors 141, 142, 143, and 144 to switch to on-states, and thereby lower the gate potential of the electronic switch 140. Thus, the electronic switch 140 is switched to an off-state. Moreover, the high voltages are converted to a feedback current by the feedback resistors 122 and 124. The PWM IC receives the feedback current via the current sense pin 130, and controls the driving voltages of the backlight lamps 111 according to the feedback current.
When an open circuit occurs in any backlight lamp 111, the corresponding sampling output 110 provides a low voltage. The low voltage causes the corresponding third diode 145 to switch to an off-state, which further causes the corresponding transistor 141, 142, 143, and 144 to switch to an off-state, respectively. The electronic switch 140 then switches to an on-state according to the DC voltage outputted from the voltage output pin 131. As a result, the current sense pin 130 of the PWM IC 13 is grounded via the electronic switch 140. Then the PWM IC 13 controls the backlight system to switch to a protecting state.
To carry out the function of protecting the backlight system, the backlight control circuit 10 needs at least five transistors 140, 141, 142, 143, and 144, four third diodes 145, and four sampling circuits 11 each including a first diode 113, a second diode 114, and a sampling resistor 115. Furthermore, if more than four backlight lamps 111 are used in the LCD, the number of transistors needed increases correspondingly. Thus, the cost of the backlight control circuit 10 is high, particularly in the case where there are a large number of backlight lamps 111.
It is, therefore, desired to provide a backlight control circuit that can be used to overcome the above-described deficiencies.